1. Field of the Invention
The present invention relates to a cap assembly comprising a gasket prevented from sagging and a cylindrical secondary battery comprising the cap assembly.
2. Description of the Related Art
Secondary batteries are classified into cylindrical, prismatic and pouch types by the shape of battery cases they employ. Generally, a cylindrical secondary battery includes an electrode assembly accommodated in a cylindrical metal can, a prismatic battery includes an electrode assembly accommodated in a prismatic metal can, and a pouch type battery includes an electrode assembly accommodated in a pouch type case made of an aluminum laminate sheet.
The electrode assemblies accommodated in the battery cases are power generating devices capable of repeated charge-discharge cycles. Each of the electrode assemblies has a stack structure of a positive electrode, a separator and a negative electrode. Electrode assemblies are classified into jelly-roll and stack types by their structure. A jelly-roll type electrode assembly is constructed by interposing a separator between long sheet-like positive and negative electrodes, to which respective active materials are applied, and winding the laminate. A stack type electrode assembly is constructed by sequentially stacking a plurality of electrode units, each of which includes a positive electrode having a predetermined size, a negative electrode having a predetermined size and a separator interposed between the electrodes. Jelly-roll type electrode assemblies are most widely used in secondary batteries due to their advantages, including ease of construction and high energy density per unit weight. Jelly-roll type electrode assemblies are commonly employed in cylindrical batteries.
A jelly-roll type electrode assembly undergoes repeated expansion and contraction during charge and discharge, and as a result, it tends to deform. In the course of the charge and discharge, stress is concentrated at the central portion of the electrode assembly to cause the electrodes to penetrate the separator and is in contact with a central metal pin, resulting in internal short circuits. Heat caused by the short circuits decomposes an organic solvent present in the battery to evolve gas. The gas increases the internal pressure of the battery, resulting in rupture of the battery. Further, the internal pressure of the battery may increase when internal short circuits are caused by an external impact applied to the battery.
Attempts have been made to solve the safety problems of batteries. For example, a cap assembly of a cylindrical battery is known in which a safety vent for exhausting high-pressure gas, safety devices, such as a positive temperature coefficient (PTC) thermistor for interrupting current at high temperature and a current interrupt device (CID) for interrupting current when the internal pressure of the battery increases, a top cap forming a protruding terminal to protect the safety devices, etc. are fixed together by a main gasket.
In the structure of the cap assembly, the main gasket surrounds the outer circumferences of the safety vent, the PTC thermistor, the top cap, etc. to prevent an electrolyte present in the battery from leaking out of the cap assembly. So long as the electrolyte does not leak through the interface between the safety vent positioned at the innermost portion of the battery and the main gasket surrounding the outer circumference of the safety vent, no electrolyte leakage occurs through the interfaces between the metallic parts, such as the interface between the safety vent and the PTC thermistor and the interface between the PTC thermistor and the top cap.
However, a portion of the electrolyte substantially leaks through the interface between the main gasket and the safety vent during charging and discharging operations of the battery or when the battery falls down or an external impact is applied to the battery. Once the electrolyte leaks through the interface between the main gasket and the safety vent, it leaks easily from the battery through the interfaces between the metallic parts. That is, due to relatively weak adhesiveness at the interfaces between the metallic parts, the electrolyte entering the interfaces between the metallic parts can leak out of the cap assembly through the interfaces between the metallic parts more easily than through the interfaces between the main gasket and the adjacent devices.
Thus, there is a strong need to develop a technique for reducing the leakage of electrolyte out of a cap assembly.
In this connection, Japanese Unexamined Patent Publication Nos. 2006-286561, 2005-100927 and 2002-373711 disclose cap assemblies, each of which includes a top cap disposed on a main gasket. However, the main gaskets surrounding the outer circumferences of safety devices have complicated shapes difficult to produce, and electrolytes leak from the interfaces between metallic parts (e.g., safety vents, PTC thermistors and top caps). That is, the cap assemblies disclosed in the patent publications fail to provide satisfactory solutions to the above-mentioned problems.
FIG. 1a is a schematic partial cross-sectional view illustrating an upper structure of a conventional cylindrical secondary battery 100.
Referring to FIG. 1a, the battery 100 is fabricated by inserting an electrode assembly 300 as a power generating device into a can 200, injecting an electrolyte into the can, and mounting a cap assembly 400 on an upper opening of the can 200. A main gasket 500 is mounted on an upper beading portion 210 of the can 200 to hermetically seal the can 200. The cap assembly 400 includes a top cap 410, a PTC thermistor 420 for interrupting an overcurrent and a safety vent 430 for decreasing the internal pressure of the battery. The elements of the cap assembly 400 are brought into close contact with each other inside the cap assembly 400.
The central portion of the top cap 410 protrudes upward. Due to this structure, the top cap 410 serves as a positive terminal to which an external circuit is connected. The top cap 410 has a plurality of through-holes (not shown) through which gas is released. The safety vent 430 has a lower end to which a positive electrode of the electrode assembly 300 is connected through a current interrupt device 440 and a positive lead 310.
The safety vent 430 is made of a thin conductive plate and has a downwardly indented portion 432 at a central portion thereof. The indented portion 432 has an upper bent portion and a lower bent portion in which two notches 434 and 436 having different depths are formed, respectively.
The current interrupt device 440 is made of a conductive plate and is installed under the safety vent 430 to interrupt current when the internal pressure of the battery increases above a critical value. The current interrupt device 440 is preferably made of the same material as the safety vent 430. An auxiliary gasket 510 is made of a polypropylene (PP) resin to prevent the current interrupt device 440 from being in electrical communication with the safety vent 430.
For example, when the battery is internally short-circuited or overcharged by various factors, the temperature of the battery 100 increases. The increased temperature leads to an increase in the resistance of the PTC thermistor 420, which greatly decreases the amount of current flowing through the PTC thermistor 420. A continuous increase in the temperature of the battery 100 decomposes the electrolyte, and as a result, gas is produced. The gas increases the internal pressure of the battery to lift the indented portion 432 of the safety vent 430, resulting in partial rupture of the current interrupt device 440. This rupture enables the current interrupt device 440 to interrupt current, thus ensuring the safety of the battery 100. When the pressure is continuously increased, the notches 436 of the safety vent 430 are ruptured. As a result, the high-pressure gas is released to the outside, thus ensuring the safety of the battery 100.
An electrolyte may leak through various portions (for example, the interface between a gasket and a safety vent) of a battery. Particularly, a main gasket surrounding a top cap, a PTC thermistor and a safety vent may not be in close contact with the safety vent at one end thereof. The elasticity of the main gasket may deteriorate during use of the battery. The inelastic portion of the main gasket may sag by gravity, and as a result, the main gasket is not in close contact with the safety vent.
It is ideal that no sagging of the main gasket occurs, as illustrated in FIG. 1a. However, in an actual case, the main gasket sags downward and is spaced from the safety vent, as illustrated in FIG. 1b. 
FIG. 1b schematically illustrates an inner portion of a conventional cylindrical secondary battery in which a main gasket sags. FIG. 1b also shows a magnified image of the sagging state of the main gasket. From the image, it can be visually observed that an end portion of the main gasket sags in the gravity direction, leaving a space between the main gasket and the safety vent.
The space formed between the main gasket and the safety vent becomes a major path through which an electrolyte present in the battery escapes.